The present invention provides a permanently antistatic, low wear, thermoplastic molding composition comprising a polybutylene terephthalate resin having an intrinsic viscosity in the range of about 0.7 to about 1.4 deciliters per gram measured in ortho-chlorophenol at 25.degree. C., an antistatic dopant comprising an electron acceptor compound, an electron donor compound, and polytetrafluoroethylene, and less than about 10 percent by weight based on the total weight of the molding composition of carbon fibers having lengths between about 0.1 and about 1.0 inch. Such a molding composition is permanently antistatic, easily made, and exhibits exceptional low wear performance, flame resistance, mechanical properties, and cost effectiveness.
Polybutylene terephthalate (PBT) resins have become widely utilized in industrial molding applications. Unlike polyethylene terephthalate, PBT resins are injection moldable at relatively low molding temperatures to form stable molded articles. Furthermore, the high rate of crystallization of PBT resins, even at the relatively low molding temperatures, greatly facilitates the mold release of articles molded from PBT resins. Articles molded from PBT resins are known to have good chemical resistance, low water absorption, and good mechanical properties.
However, PBT resins exhibit a marked tendency to become charged with static electricity. The highly electrified surfaces of articles molded from PBT resins are known to be extremely hazardous if such articles come into contact with flammable or explosive materials. Thus, the material handling industry has had a long-felt need for cost effective, low wear thermoplastics that have sufficient electrical conductivity so as to prevent the buildup of a static electric charge on material handling equipment such as moving conveyor parts. Conveyor applications that involve the movement of flammable or explosive material or sensitive electronic circuitry simply cannot tolerate the safety hazards inherent with the accumulation of a static electric charge on conveyor components. Current federal standards and technology indicates that a volume resistivity of less than 10.sup.8 ohm-cm is necessary to alleviate the problems associated with the aforedescribed buildup of static electricity.
It is known that the addition to molding compositions of carbon fibers in concentrations of 10 to 40 percent by weight of the total weight of the molding composition, improves the electrical conductivity of the compositions.
However, due to the cost of carbon fibers, such compositions are prohibitively expensive. Also, it is difficult to melt compound such compositions by extrusion or other melt compounding techniques. Such difficulty results in a non-uniform dispersion of carbon fiber in the resin matrix, poor processability, poor uniformity of properties such as highly variable mechanical properties, and poor fiber strength and modulus translation in the composite. The poor processability of these conventional compositions is due to their lack of homogeneity and high fiber content.
U.S. Pat. No. 4,024,204 discloses PBT molding compositions containing from 0.5 to 20% by weight, based on the PBT, of an alkoxylated polyamide finely distributed therein to reduce the tendency of the PBT to electrification.
U.S. Pat. No. 4,148,960 discloses a process for the production of shaped objects such as films, fibers, and fabrics, from a polymeric composition having improved antistatic properties. The composition comprises a melt blend of a polyester and a block terpolymer selected from the group consisting of lactam-polyolpolyacyl lactam and lactam-polyolacylpolylactam.
U.S. Pat. No. 3,462,185 discloses an antistatic polyester composition comprising a polyethylene terephthalate based polyester and a phosphorous acid triester derivative therein containing a polyalkylene oxide chain in the molecule.
G.B. Pat. No. 1,228,573 discloses a material which comprises 80 percent by weight of lengths of reinforcing carbon filaments and 20 percent by weight of filaments or films of thermoplastic material wherein the filaments and/or films have been coalesced together under the influence of heat and pressure.
U.S. Pat. Nos. 4,369,280, 4,373,067, 4,283,326, 4,195,011, and 4,107,231 all disclose compositions comprising inter alia a polyester resin, carbon fibers, and various flame retardant additives.
According to the present invention, a cost effective, easily processed, thermoplastic molding composition that exhibits excellent low wear performance, flame resistance, and mechanical properties, comprises a PBT resin, an antistatic dopant comprising an electron acceptor compound, an electron donor compound, and polytetrafluoroethylene, and less than about 10 percent by weight of the total composition of carbon fibers having lengths between about 0.1 and 1.0 inch, is permanently antistatic in that the composition is characterized by a volume resistivity of less than 1.times.10.sup.8 ohm-cm.
Therefore, it is an object of the present invention to provide a thermoplastic molding composition that exhibits a volume resistivity of less than 1.times.10.sup.8 ohm-cm.
It is an object of the present invention to provide a permanently antistatic thermoplastic molding composition suitable for use in handling flammable or explosive materials or sensitive electronic circuitry.
It is a further object of the present invention to provide a relatively low cost permanently antistatic thermoplastic molding composition.
It is also an object of the present invention to provide a relatively low cost, permanently antistatic thermoplastic molding composition that exhibits exceptional wear performance, flame resistence, and mechanical properties.
It is a further object of the present invention to provide a thermoplastic molding composition suitable for handling material that cannot tolerate the buildup of a static electric charge on the handling equipment.
It is also an object of the present invention to provide a relatively low cost, permanently antistatic thermoplastic molding composition with good processability, i.e., ease of injection molding into complex shapes.